Natural gas liquefaction is practiced at remote natural gas-rich locations to convert the gas to a transportable liquid for shipment to markets in energy-poor areas. It is desirable to minimize the specific energy consumption for producing liquefied natural gas (LNG) in order to reduce the final product cost and/or increase the profitability of LNG production. Alternately, it becomes necessary at certain times to maximize LNG production while consuming the least amount of energy possible at maximum production rates. The liquefaction of natural gas at cryogenic temperatures requires one or more energy-intensive refrigeration systems, and the proper control of such refrigeration systems is critical to minimize energy consumption or maximize the amount of LNG produced.
Feedback control systems are widely used to achieve efficient operation of LNG plants by controlling the perturbations normally encountered in the operation of such large and complex process plants. Such perturbations occur for example due to upsets in the operation of certain equipment in the plant, adjustments of operating conditions by plant operators, changes in production rates, and the like. In these feedback control systems, a plurality of parameters including pressure, temperature, flow rate, composition, and liquid level at specific locations in the process are controlled at desired set points by measuring each parameter, determining the deviation of each parameter from its set point, and using the value of the deviation to control a piece of equipment (for example a valve) at a location elsewhere in the process in order to minimize the deviation of each measured parameter from its set point. The specific hardware and software utilized in such feedback control systems are well known in the field of process plant control, see for example Chemical Engineers' Handbook. Fifth Edition, McGraw-Hill, pp. 22-1 to 22-147.
U.S. Pat. No. 3,742,721 discloses a control method for a gas liquefaction process in which the temperature on the refrigerant side of the main liquefaction heat exchanger is controlled by adding appropriate amounts of makeup refrigerant to the recirculating refrigerant stream. In addition, the pressure at the same location is controlled by regulating the speed of the refrigerant compressor. Other feedback control loops controlling temperatures, pressures, and liquid levels are also described.
The basic concepts of operating an LNG plant computer control system is described in an article by B. G. Tompkins entitled "LNG Plant Computer System: a Conceptual Philosophy" in the Oil and Gas Journal. pp.57-60, Nov. 26, 1979. The author covers the important factors in designing such a system and reviews the various types of hardware and software which can be utilized in the system.
U.S. Pat. No. 4,457,768 discloses the control of a refrigeration process used to liquefy natural gas in which the liquid level in one or more refrigerant flash drums is maintained by controlling the rate(s) of liquid flow from the drum(s). The liquid refrigerant level in the main liquefaction heat exchanger is maintained by regulating the flow rate of the recirculating refrigerant across a pressure letdown valve.
A series of Soviet Union patents SU 1,043,442, 1,354,007, 1,357,662, and 1,458,663 disclose various feedback control systems for operating LNG plants including loops for controlling flow rates, liquid levels, pressures, temperature differentials, and actual temperatures.
An automated control system for a multicomponent refrigeration system used for liquefaction of natural gas is disclosed in U.S. Pat. No. 4,809,154. The system includes a plurality of sensors for detecting various conditions in the plant such as temperature, pressure, flow, and composition, a plurality of controllers such as servo-controlled valves, and a computer for executing the control program. The control system operates the plant to provide the desired production rate with the highest possible efficiency, or maximizes the production rate while attaining the highest possible efficiency at that production rate. The control system responds to changes in plant conditions and then adjusts various pieces of equipment to eliminate excursions from desired controller set points; the control system therefore operates in a feedback mode.
Many LNG plants are located at remote sites where the refrigerant compressors are driven by steam turbines using steam generated onsite or by gas turbines fired with a portion of the plant feed gas and/or product gas. Gas turbines are preferred over steam turbines in many locations for their potential lower overall cost. Gas turbines have the disadvantage, however, that power output can be affected by ambient air temperature. An increase in ambient temperature has two potential effects on LNG plant performance: (1) if the turbine is not operating at maximum available power, an increase in the fuel rate to the turbine combustor is required to provide the necessary power to maintain LNG production; or (2) if the turbine is operating at maximum available power, the turbine power output will decrease and LNG production will likewise decrease. A drop in the ambient temperature typically causes the opposite response wherein the gas turbine available power output increases. In addition, when the plant site requires the use of ambient air cooling for refrigerant compressor intercooling and aftercooling, the heat rejection from the refrigeration system is reduced when ambient temperature rises, which in turn reduces the amount of available refrigeration for gas liquefaction.
Since ambient air temperature usually cycles through a minimum and a maximum each day, or can be subject to wide swings due to severe weather changes, the optimization of a gas liquefaction process is not possible using the various feedback control systems described in the background references cited above. The invention disclosed in the following specification and defined in the appended claims solves this problem and allows the optimum operation of an LNG plant during dynamic ambient temperature changes at the plant site.